The present invention relates to a magnetic recording medium used for a recording device which records/reproduces information into/from a recording medium by means of magnetization while a temperature thereof is being raised.
In the field of magnetic recording techniques, the following recording/reproducing method has been used generally. In this recording/reproducing method, a ferromagnetic material, such as CoCrTa, is used as a magnetic film for the magnetic recording medium. Information is recorded into the medium by applying an external magnetic field from a recording magnetic head, and reproduced from the medium by detecting the direction of magnetization therein using a reproducing magnetic head.
Also, another recording/reproducing method as follows has been proposed. In this recording/reproducing method, a ferromagnetic material, such as CrO2, is used as a magnetic film for the magnetic recording medium. Information is recorded into the medium by applying an external magnetic field from a recording magnetic head while a coercive force therein is being reduced with irradiation of laser beams, and reproduced from the medium in the same manner as the firstly-mentioned method, that is, by detecting the direction of magnetization therein using a reproducing magnetic head.
However, in both of the above recording/reproducing methods, the width of a recordable/reproducible track is limited by the track width of the magnetic head, and therefore, narrowing the track pitch can increase recording density only to a certain extent.
In order to eliminate the above inconvenience, still another recording/reproducing method is disclosed in, for example, Japanese Patent Gazette, Patent No. 2617025 (date of registration: Mar. 11, 1997). In this recording/reproducing method, a ferrimagnetic material having a compensation point around room temperature is used as a magnetic film for the magnetic recording medium. Information is recorded into the medium by applying an external magnetic field from a recording magnetic head while the temperature is being raised in a target recording track (into which information will be recorded), and reproduced by detecting the direction of magnetization in a target reproducing track (from which information will be reproduced) while the temperature thereof is being raised.
The magnetization in the magnetic recording medium used in this recording/reproducing method has temperature dependence such that becomes nearly xe2x80x9c0xe2x80x9d (zero) at room temperature, and increases with rising temperatures until it reaches its maximum at a finite temperature, and then drops to xe2x80x9c0xe2x80x9d again at a temperature as high as or higher than the Curie temperature. Also, the coercive force of the magnetic recording medium has a large value at room temperature, and it monotonously decreases with rising temperatures until it reaches 0 at the Curie temperature.
By raising the temperature in a target recording track of the magnetic recording medium with heating means, such as laser beams, the coercive force decreases only where the temperature is raised and becomes smaller than the magnetic field applied from the magnetic recording head. Therefore, by exploiting the above characteristics, information is recorded into a domain heated by the laser beams or the like, that is, a domain which is narrower than the width of the recording magnetic head.
As to reproducing information, by raising the temperature in a target reproducing track of the magnetic recording medium, the magnetization is increased only where the temperature is raised. Here, because the temperature in the adjacent track remains at or around room temperature, substantially no magnetic flux leaks from the adjacent track. Consequently, a signal can be detected from a track narrower than the width of the reproducing magnetic head.
According to the above recording/reproducing method, information can be recorded/reproduced at high density regardless of the track widths of the recording and reproducing magnetic heads.
In order to obtain a stable reproducing signal, the above recording/reproducing method requires the following conditions: (1) the magnetization is 0 or almost 0 at room temperature; and (2) the magnetization in the heated domain is stable, that is, the magnetization should not change at or around a temperature above room temperature, at which the magnetization reaches its maximum. However, the magnetization of the magnetic recording medium in actual use changes sharply with increasing temperatures at or around the temperature at which the magnetization reaches its maximum. Therefore, it has been difficult to obtain a stable reproducing signal.
The above inconvenience can be eliminated by controlling the temperatures in a precise manner, but this demands an expensive temperature raising device equipped with a controller. Moreover, it takes a time to adjust and maintain a temperature at a certain level, thereby presenting a problem that information can not be reproduced at a high speed.
It is therefore an object of the present invention to provide a magnetic recording medium, into which information is recorded by applying an external magnetic field from a recording magnetic head while a temperature is being raised in a target recording track (into which the information will be recorded), and from which information is reproduced using a reproducing magnetic head while a temperature is being raised in a target reproducing track (from which the information will be reproduced), and by making temperature dependence of magnetization gradual, the magnetic recording medium outputs a reproducing signal that remains substantially stable in response to a change in temperature while the temperature is being raised.
To fulfill the above and other objects, a magnetic recording medium of the present invention is characterized by comprising:
at least two magnetic layers, both having their respective magnetic compensation temperatures around room temperature, each having different magnetization, coercive force, and temperature characteristics,
the magnetic layers being magnetically coupled to each other, wherein
information is recorded into the magnetic recording medium by applying an external magnetic field from a recording magnetic head while a temperature is being raised in a target recording track, and information is reproduced from the magnetic recording medium using a reproducing magnetic head while a temperature is being raised in a target reproducing track, from which the information will be reproduced.
Conventionally, the magnetic recording medium uses a single-layer magnetic layer. However, because such a single-layer magnetic layer is generally made of rare earth elements having sharp temperature dependence of magnetization, the magnetization changes significantly in response to a change in temperature while the temperature is being raised. In contrast, according to the arrangement of the present invention, because the magnetic layer has at least two magnetically coupled layers, each having different magnetization, coercive force, and temperature characteristics, the temperature dependence of magnetization in the layer is made gradual as a whole. Also, by adequately selecting the kinds of magnetic layers, it has become possible to provide a magnetic recording medium having optimal magnetization, coercive force, and temperature characteristics in recording/reproducing information.
Here, xe2x80x9cmagnetically coupledxe2x80x9d means either magnetostatic coupling or exchanged coupling. Magnetostatic coupling is defined as coupling by means of a Coulomb force, and exchanged coupling is defined as strong coupling of atoms by a quantum mechanical force.